The nameif command gives the interface a name and assigns a security level. Typical names are outside, inside, or DMZ.

Security levels are numeric values, ranging from 0 to 100, used by the appliance to control traffic flow. Traffic is permitted to flow from interfaces with higher security levels to interfaces with lower security levels, but not the other way. Access-lists must be used to permit traffic to flow from lower security levels to higher security levels. The default security level for an outside interface is 0. For an inside interface, the default security level is 100.

The nat statement, as shown above, tells the firewall to allow all traffic flowing from the inside to the outside interface to use whatever address is dynamically (DHCP) configured on the outside interface.

In the route statement at the end if you use the word interface instead the traffic is pat to firewall interface ‘route outside 0.0.0.0 0.0.0.0 interface’